Annealing process



Feb. 14, 1939. J. L. PEARSON ANNEALING PROCESS Filed Dec. 5, 1934 2 Sheeis-Sheet 1 Ella-v, Linzfiwpadidn Feb. 14, 1939. J, PEARSON 2,146,760

' ANNEALING PROCESS Filed Dec. '5, 1954 2 Sheets-Sheet 2 DOOR OPENING I N INCHES I I I 0 I /2 4 s 5 ID PER CENT OXYGEN John, Lindon Pearson Patented Feb. 14, 1939 UNITED. STATES.

PATENT OFFICE 2,146,760 ANNEALING raocE ss Application December 5, 1934, Serial No. 756,185 In Great Britain December 8, .1933

8 Claims. (Cl. 14813.1)

This invention relates to improvements in annealing processes in furnaces in which a neutral or reducing atmosphere is maintained in the annealing furnace, in order to pre'ventscaling or 6 oxidizing of the surfaces of metals being annealed therein.

Considerable attention has been paid during recent years to this art, and more particularly to that branch known as bright annealing, which offers many advantages over the older methods of heat treatment in oxidizing atmospheres followed by pickling or descaling. The main factor which has prevented the wider adoption of the use of neutral or reducing atmospheres in connection with the heat treatment of metals and their products has been the cost of the gas necessary to produce this atmosphere. This is especially the case with continuous annealing furnaces and with relatively cheap metal products.

In continuous furnaces using protective atmosperes in which the articles to be treated move through the furnace in a continuous stream, 1

it is not possible to seal the ends against leakage of protective atmosphere and infiltration of air. Hitherto the former has been allowed to escape from the ends at a rate sufficient to prevent air leaking into the furnace up to any point where the temperature is such that oxidation would take place. Consequently, the quantity of gas required becomes so great in the case of large annealing furnaces that in many cases the process is considered to be uneconomical with known methods.

In batch furnaces during the actual period of heating, there is generally no difficulty with air contamination, since they can be effectively sealed from the outside atmosphere. However, when a batch furnace is closed after being charged with articles to be heat treated, it contains air which has entered during the charging operation and its oxygen has to be removed before heat treatment can be commenced. Up to the present this removal has been effected by sweeping it out into the outside atmosphere by a such as carbon dioxide, which is in turn swept out by a stream of protective gases. Although a considerable proportion of the air is thus removed en masse, mixing between the protective gas or special gas and the air takes place to a mosphere and wasted before the furnace is suffistream of protective gases or by a special gas ciently free from oxygen for heat treatment to commence.

The object of the present invention is to reduce the consumption of protective atmosphere in both continuous and batch furnace processes of thermal treatment.

According to the present invention, the consumption of protective atmosphere is decreased in the case of continuous furnaces by' recovering at least in part the protective atmosphere leaving the heat treatment zone of the furnace, and utilizing the adventitious oxygen, present in the protective atmosphere through air infiltration from outside the furnace, for the production of further amounts of protective atmosphere by. combustion with combustile gas, the resulting protective atmosphere being recirculated to the furnace; and in the case of batch furnaces, by driving the air left in the furnace after charging with the articles to be heat treated, at least in part into'the protective atmosphere circulating system, and utilizing it for the production of further quantities of protective atmosphere by combustion with combustible gas.

In the case of furnaces of the continuous type, the withdrawal of the gases is most conveniently effected by means of one or more ports in the inlet and outlet ducts where the temperature has fallen sufliciently low that the materials being bright annealed may be safely exposed to an atmosphere containing oxygen.

The removal of the oxygen from the withdrawn gases is carried out in apparatus (hereinafter referred to as a burner) in which the gases withdrawn from the furnace are heated (in the presence of a catalyst for at least a portion of the time) in admixture with butane, natural gas, decomposed ammonia or the like, or their products of combustion, with a restricted amount of oxygen-containing gas, and for a portion, at

least, of the period of heating, in the presence of a catalyst, whereby oxygen contained in the gases withdrawn from the furnace is removed.

The gases leaving the burner may be reduced to the temperature required in known forms of coolers, and usually it is desirable to provide means for separating water from the gases before they are returned to the furnace.

In order to obtain reasonably smooth operation of the burner, it is desirable to ensure that the gases which are sent to it have a fairly uniform'oxygen content. This may be achieved in the case of continuous furnaces by providing a number of curtains of soft material such as asbestos cloth in the inlet and dischar e ducts which control the ingress of air and egress of reducing gases through the openings, even when varying sizes of articles are passing in and out of the furnace. It is also an advantage to provide a'capacity vessel in the pipe line leadingfrom the furnace back to the regenerative burner. Furthermore the amount of oxygen in the protective gas from the furnace may notbe sufficient when burnt with ammonia, natural gas,

or the like, or their partial combustion products,

In this case.

to render the burner autothermal. autothermicity may be obtained by adding further oxygen, e. g., from the outside atmosphere, to the mixture of contaminated protective gas with ammonia, natural gas or the like, or their partial combustion products, and burning it in the burner.

The gas or gases withdrawn from the furnace may contain objectionable constituents arising from the lubricants used in cold working as for example in tube drawing, or wire drawing, and this objectionable constituent may be removed either before or after the regenerative burner by means of suitable scrubbing or purification means. Thus for example, unless the tubes or wire are degreased before bright annealing, the gas withdrawn from the furnace may contain hydrogen sulphide, and this can be removed either by passing the gas through the ordinary type of iron oxide box or by scrubbing the gas in a small tower or washer by means of a solution of sodium carbonate or other liquor.

The application of the present invention to annealing processes in furnaces of the continuous type will now be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic elevation of an annealing furnace showing the gas circulation system, and Figure 2 is a graph showing the effect of different kinds of material when used for screening the inlet and outlet passages of the furnace.

Referring to Figure l, the neutral or reducing gas, consisting for example of nitrogen or nitrogen containing a small percentage of hydrogen, is fed into the tunnel-like annealing furnace I through one or more ports 2 situated in the wall of the furnace where the cooling chamber 3 adjoins the heating chamber ,4. The rateat which the gas is introduced is measured by means of the fiowmeter 5, and is sufficient to produce in the inlet and outlet ducts 6 and I for the articles being annealed a velocity such-that ingress of oxygen from the outside atmosphere is, prevented from reaching the metal articles before they are cooled to a safe temperature.

A band conveyor 8 carries the metal objects to be heat treated, e. g., tubes, bars, or wire, through the furnace, and flexible curtains 9 and H] are arranged to screen the inlet and outlet ducts at their outer ends. Gas exit ports H and I 2 are provided in the gas ducts 6 and 1. Connections for manometers l3 and I4 are provided to enable the pressure of the gas at these points to be measured with a view to controlling the flow of gas by means of valves l5 and I6. The ends of the furnace are fitted with vertical sliding doors I! and I8, to which are attached inclined baflles l9 and 20.

The gas which is withdrawn from the furnace through ports II and I 2 passes via pipes 2| and 22 to a commonmain 23 leading to a capacity vessel 24 and a suction fan 25. The fan delivers the gas through a valve 26 to a heat exchanger 21'. The gas enters the coil 28, in which 'it is preheated, and emerges by the pipe 29 leading to a catalyst combustion chamber 30.

The gas issuing from the pipe 29 is mixed in the chamber 30 with additional gas entering by pipe 3|. This additional gas is derived from the decomposition of ammonia which is introduced by pipe 32 into a coil 33 in the vessel 34 adjoining the catalystchamber 30. The coil 33 is heated in a manner to be explained and the ammonia is partially or completely decomposed during its passage through the coil. From the coil the gas passes through pipe 35 to a burner 36 where it is mixed with air extracted from near the ends of the furnace l by means of the fan 31, the gas delivery rate of which is measured by a fiowmeter 48. This air contains a certain amount of valuable gas which would otherwise escapefrom the furnace and which is recovered by the arrangement shown. However, this arrangement is optional and it is possible to operate the burner with ordinary air. The gases issuing from the burner are ignited and burn in the space 38 within coil 33, thus heating the latter and causing the decomposition of the ammonia passing through the same.

The combustion products pass through pipe 3| and mix with the gas issuing from pipe 29 as previously described. The mixed gases then pass over the catalyst gauzes 38, 39 and 40 in succession and their oxygen content is substantially eliminated. The gauze 38 is made of an iron alloy containing 18% chromium, 8% nickel, 1% tungsten and 1% vanadium; the gauge 39 is made of copper and the gauge 40 is made of palladium.

The resulting gases pass via pipe 4| into the heat exchanger 21 and are caused to sweep over the coil 28 by the batllc 42 which prevents their passing straight through the space surrounded by the coil. The gases then pass by pipe 43 to a cooler 44 and thence to a drier 45 and a capacity vessel 46 before being returned to the furnace by pipe 41 leading to the port 2.

During the starting up period all the neutral or reducing gas may be allowed to pass out through the ends of the ducts 6 and 1 until the atmosphere in the furnace is substantially free of oxygen. The recirculating fan 25 is now started and the valves [5 and I6 adjusted until the rate, measured by the fiowmeter 5|, at which gas is withdrawn from the furnace, is about of the rate of introduction of gas into the furnace via the pipe 41. About 5% of the gas introduced into the furnace is passed out to atmosphere through each of the inlet and outlet ducts, and this escaping gas together with the curtains 9, I 0 and baffles I8, 20. minimizes the entrance of oxygen due to diffusion and/or natural draughts. By means of the sampling points 43, 58 the composition of the gases leaving and passing to the furnace may be tested and the percentage of gases withdrawn adjusted to give the correct atmosphere in the furnace.

In the case of a small annealing furnace with inlet and outlet ducts of 0.44 square feet cross sectional area and a total furnace length of 10 feet, a gas mixture consisting of nitrogen and 5% hydrogen by volume was fed into the furnace at the rate of 230 cubic feetper hour. The rate at which gas was withdrawn from the furnace by the recirculating fan was about 218 cubic feet per hour, and the rate at which makeup gas was supplied was 12 cubic feet per hour. With an oxygen content of 2 per cent by: volume this oxygen and also to produce a make-up gas containing 10% hydrogen by volume. The amount of air provided by the air blower for the make-up gas was 12.5 cubic feet per hour. This air or mixture of air and gas was taken from points near the furnace ends external to the curtains 9, ID in order to minimize the effect of eddy currents or draughts of air in causing contamination of the furnace gas with oxygen.

Under these conditions and with inlet and outlet passages without curtains or inclined baffles fitted to the doors, the degree of oxygen contamination is indicated by the curve A in Figure 2. Curve .B in Figure 2 shows the improvement obtained by fitting the inclined baflies to the doors. Curve C in, Figure 2 shows the effect of fitting divided curtains in addition to the inclined baiiles. Curve D in Figure 2 shows the effect of substituting string curtains for the divided curtains, the inclined baiiies being retained. Objects of various sizes could be passed in and out of the annealing furnace and no variation in the degree of oxygen contamination could be detected.

I claim:

1. In a process of bright thermal treatment of metal in a protective atmosphere in a furnace adapted for the continuous passage of metal via inlet and outlet ducts, said furnace having a heat treatment zone supplied with a continuous stream of non-oxidizing g'as forming said protective atmosphere which flows outwardly along both of said inlet andoutlet ducts and is there subject to contamination with air, the steps of recovering at least a portion of the contaminated gas flowing through said ducts, combusting the oxygen content of the recovered gas with a combustible gas, and returning the resulting nonoxidizing gas to the heat treatment zone of the furnace.

2. In a process of bright thermal treatment. of metal in a protective atmosphere in a furnace adapted for the continuous passage of metal via inlet and outlet ducts, said furnace having a heat treatment zone supplied with a continuous stream of non-oxidizing gas forming said protective atmosphere which flows outwardly along both of said inlet and outlet ducts and is there subject to contamination with air, the steps of recovering at least a portion of the contaminated gas flowing through said ducts, mixing the recovered gas with an amount of combustible gas at least suflicient to combine with the entire oxygen content of the recovered gas, subjecting the mixture of recovered gas and combustible gas in the presence of a metallic catalyst to a temperature adapted to cause combination of the oxygen and the combustible gas, and returning the resulting gas to the heat treatment zone of the furnace.

3. In a process of bright thermal treatment of metal in a protective atmosphere in a furnace adapted for the continuous passage of metal via inlet and outlet ducts, said furnace having a heat treatment zone supplied with a continuous stream of non-oxidizing gas forming said protective atmosphere which flows outwardly along both of said inlet and outlet ducts and is there subject to contamination with air, the steps of recovering at least a portion of the contaminated gas flowing through said ducts, mixing the recovered gas with an amount of combustible gas at least suflicient to combine with the entire oxygen content of the recovered gas, said combustible gas being obtained by heating ammonia to its decomposition temperature and combiusting the resulting gas with a restricted amount of air,

subjecting the mixture of recovered gas and comcombination of the oxygen and the combustible gas, and returning the resulting gas to the heat treatment zone of the furnace.

4. In a process as set forth in claim 1, preheating the recovered gas, prior to mixing it with the combustible gas, by means of the sensible heat of the combusted gas about to be returned to the heat treatment zone of the furnace.

5. In a process as set. forth in claim 3, withdrawing air from near the outer ends of the inlet and outlet ducts of the furnace, said air containing a proportion of the non-oxidizing gas forming the protective atmosphere within the heat treatment zone, and utilizing said airfor the partial combustion of the decomposed ammonia.

6- In a process of bright thermal treatment of metal in a. protective atmosphere formed of gas prepared by the burning of combustible gas under conditions so controlled as to reduce the oxygen content of the resulting protective gas to an innocuous value; the procedure for-producing and maintaining an atmosphere of the desired protective gas in a furnace chamber having ends for ingress and egress of material to be treated and while recovering, at least in part, the protective gas leaving the chamber, which comprises driving the protective gas into the chamber and with-' drawing the protective gas contaminated with air from the chamber into a closed circulating system, supplying to the system from an extraneous source and mixing with said protective gas and air a reducing make-up gas such as ammonia, butane, natural gas, coal gas or the like suitable for producing by combustion the desired protective gas, passing the mixed gases with a controlled amount of air, taken from near the ends of the furnace, into a combustion chamber maintained at a temperature sumcient to cause the oxygen content of the mixture to combine with the reducing constituents in the mixed gasesand thereby utilizing the oxygen contained in the air withdrawn from the chamber and in the air taken from near the ends of the furnace into the system for combustion with the make-up gas introduced into the system to produce the desired protective gas.

7. In a process of bright thermal treatment of metal in a protective atmosphere formed of gas prepared by the burning of combustible gas under conditions so controlled as to reduce the oxygen content of the resulting protective gas to an innocuous value; the procedure for producing and maintaining an atmosphere of the desired protective gas in a furnace chamber having ends for ingress and egress of material to be treated and while recovering, at least in part, the'protective gas leaving the chamber; which comprises driving the protective gas into the chamber and withdrawing the protective gas contaminated with air from the chamber into a closed circulating system, supplying to the system from an extraneous source and mixing with said protective gas and air a reducing make-up gas such as ammonia, butane, natural gas, coal gas or the like suitable for producing by combustion the desired protective gas, passing the mixed gases with a controlled amount of an oxygen-containing gas into a combustion chamber maintained at a temperature sufiicient to cause the oxygen content of the mixture to combine with the reducing constituents in the mixed gases and thereby utilizing the oxygen contained in the air withdrawn from the chamber and in the oxygen-containing gas drawn into the system 'for combustion with the make-up gas introduced into the system to produce the desired protective gas, the gases leaving the combustion chamber being used to pre-' heat the gas withdrawn from the furnace chamber on its way to the combustion chamber.

8. In a process for the heat treatment of metals in a protective atmosphere, in which said atmos- 10 phere comprises the gaseous products of partial JOHN LINDON PEARSON. 10 

